1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus and an image pickup system.
2. Description of the Related Art
In a solid-state image pickup apparatus using a CMOS-type image sensor, it is known to provide a focal plane electronic shutter function. The electronic shutter function refers to a function to reset a signal charge storage region in a photodiode in each pixel before the photodiode starts to store a signal charge generated via a photoelectric conversion thereby to achieve the ability to change the charge accumulation period of the photodiode. In the focal plane electronic shutter, a large number of pixels arranged in a two-dimensional array are scanned and reset sequentially from one pixel row to the next in synchronization with a horizontal synchronization signal. After a particular period of time has elapsed, signal charges stored in pixels are sequentially read row by row. The “particular period of time” described above is called a charge accumulation period of the photodiode, and the charge accumulation period is equal for any row as long as the intervals of the horizontal synchronization signal (horizontal period) are maintained constant. On the other hand, as described in Japanese Patent Laid-Open No. 2004-023615, to remove flicker noise, it is known to adjust the length of one vertical period by providing a fraction adjustment period and changing the length of the horizontal period by changing a count value of a fraction count.
Suppose a case where the adjustment of the length of one vertical period by changing the horizontal period by providing the fraction adjustment period is performed in a solid-state image pickup apparatus having the focal plane electronic shutter function, as described in Japanese Patent Laid-Open No. 2004-023615. In this case, a difference can occur in charge accumulation periods between a region including a fraction adjustment period and a region including no fraction adjustment period. If the difference in length between the fraction adjustment period and the normal period increases, this results in an increase in difference in pixel signal levels between the region in which the charge accumulation period includes a fraction adjustment period and the region in which there is no fraction adjustment period, which causes a problem of degradation in image quality.